Final answer:
Chromium (Cr) is reduced in the reaction, transitioning to a lower oxidation state in the product chromium(III) oxide (Cr2O3).
Step-by-step explanation:
In the provided reaction, chromium (Cr) is the species that has been reduced. Reduction in chemistry refers to the gain of electrons by a species. Looking at the reaction, chromium starts in an unspecified oxidation state and ends in the chromium(III) oxide (Cr2O3) compound, where it has an oxidation state of +3. This indicates that chromium's oxidation state has decreased, which is the definition of reduction. Furthermore, since oxygen is gaining electrons, it is typically the species being reduced; however, in this reaction, oxygen starts in the O2 molecule with an oxidation state of 0 and ends in the compound Cr2O3, where each oxygen has an oxidation number of -2. Thus, oxygen has not gained electrons in this case, confirming that chromium is the species being reduced.